Different measurement principles are known for the determination of the flow speed or of the flow rate on an ultrasound basis. In a Doppler process, the frequency shift of an ultrasonic signal reflected at the flowing fluid that differs in dependence on the flow speed is evaluated. In a differential transit time process, a pair of ultrasonic transducers is mounted with a mutual offset in the longitudinal direction at the outer periphery of the conduit, said pair of ultrasonic transducers transmitting and registering ultrasonic signals alternatingly transversely to the flow along the measurement path spanned between the ultrasonic transducers. The ultrasound signals transported by the fluid are accelerated or decelerated by the flow depending on the running direction. The resulting transit time difference is calculated using geometrical parameters to form a mean flow speed of the fluid.
The volume flow or flow rate results from this with the cross-sectional area. For more exact measurements, a plurality of measurement paths each having a pair of ultrasonic transducers can also be provided to detect a flow cross-section at more than one point.
The ultrasonic transducers used to generate the ultrasound have an oscillating body, frequently a ceramic material. With its aid, an electric signal is, for example, converted into ultrasound, and vice versa, on the basis of the piezoelectric effect. Depending on the application, the ultrasonic transducer works as a sound source, as a sound detector or as both. In this respect, a coupling has to be provided between the fluid and the ultrasonic transducer.
A widespread solution comprises allowing the ultrasonic transducer to project into the line with a direct contact to the fluid. Such intrusive probes can make exact measurements more difficult due to a disturbance of the flow. Conversely, the dipping ultrasonic transducers are exposed to the fluid and to its pressure and temperature and are thereby possibly damaged or lose their function due to contamination and deposits.
Techniques are generally also known in which the inner wall remains completely closed. One example is the so-called clamp-in assembly, for instance in accordance with U.S. Pat. No. 4,467,659 by which ultrasonic transducers are fastened to the line from the outside. However, only diametrical measurement paths can thus be implemented through the pipe axis, whereby additional errors are generated with non-axially symmetrical flow profiles. In addition, the signal-to-noise ratio is reduced due to the solid pipe wall thickness the ultrasound signal has to penetrate and the measurement system thereby becomes more prone to disturbance.
JP 2000 337 940 A shows a further flow rate measurement apparatus in which the piezoelectric elements contact the line wall at the bottom of a bore in the line. However, a complicated multi-part design is required for this purpose and the probes can disturb the flow. The problems of a robust measurement, of a sufficiently broad radiation, and of a simple transducer design are consequently not solved.
It is proposed in DE 102 49 542 A1 to attach the ultrasonic transducer directly to a functional surface that is in contact with the medium. A path alignment having a component in the flow direction is achieved by chamfering the functional surfaces and thus of the line. A planar, unimpeded inner pipe wall is thereby precluded.
EP 1 378 727 B1 proposes attaching the ultrasound-generating elements to an outer side of a wall. Unlike the clamp-on technique, the ultrasonic transducer is in this respect so-to-say integrated into the wall. A pocket having a substantially smaller wall thickness than the remaining wall is formed in the region of the ultrasonic transducers and the remaining wall thickness forms the membrane of the ultrasonic transducer. This assembly, also known as clamp-in, is so-to-say an intermediate form of a fixed assembly in the inner space of the line and the clamp-on assembly. The transducer design is, however, relatively complicated and a broad, almost spherical radiation is at least not secured for higher frequencies with a small radiation surface.
In principle, an ultrasound flowmeter having a smooth inner pipe wall and having ultrasonic transducers integrated into the pipe wall would solve most of the previously described problems. To simultaneously achieve a simple design of the ultrasonic transducers and a sufficiently large radiation surface, a complex geometry of the pockets in the pipe wall in which the ultrasonic transducers are arranged is necessary.
However, this is preceded by the practical challenge of producing such a pocket with a high precision and simultaneously inexpensively. In principle, filigree structures could be produced by micro cutting, with micro cutters having a diameter of less than one millimeter being used in dependence on the structure sizes to be produced. For this purpose, however, high machining times are necessary and in addition the tools wear fast with materials that are difficult to cut. Other processes that can initially be considered are micro cutting in combination with microwelding, micro-EDM (electric discharge machining), micro-ECM (electrochemical machining) or laser ablation. These processes are, however, neither economic nor sufficiently precise.
A further production process known per se that has, however, not previously been associated with ultrasound measurement or even with the manufacture of transducer pockets in the prior art is profile drilling. This is drilling using a profile die to generate rotationally symmetrical inner surfaces that are determined by the main cutting edge profile of the tool. Tools from a diameter of more than 3 mm onward are primarily used to produce profile bores. Profile drilling would be superfluous for the simple geometry of a transducer pocket as in EP 1 378 727 B1.
DIN 8589 allocates profile drilling to the cutting processes in the subgroup of drilling/countersinking/reaming that in turn belongs to the group of cutting using a geometrically specified cutting edge. Subvariants of profile drilling include:                Profile drilling into solid material. Drilling into the solid material to produce the rotationally symmetrical profiled bores determined by the main cutting edge profile of the drilling tool.        Profile reboring. Reboring an already existing or preworked hole to generate the rotationally symmetrical inner surface determined by the main cutting edge profile of the drilling tool.        Profile countersinking. A drilling process carried out using a profile countersinking tool for generating rotationally symmetrical, profiled countersinks determined by the main cutting edge profile of the drilling tool.        Profile reaming. Profile reboring with a small cutting thickness by a reaming tool for generating dimensionally accurate and true-to-shape, profiled inner surfaces of high surface quality.        
Against this background, it is the object of the invention to improve the arrangement of ultrasonic transducers in an ultrasound flow rate measurement apparatus.